While the position of a mobile object can be calculated based upon GPS signals, the mobile object position may need to be updated while no GPS signals are being received, e.g., while the mobile object is traveling in a shielded condition. This issue is addressed by determining in advance a distance factor indicating a relationship between the tire rotation rate and the travel distance and calculating the travel distance based upon information pertaining to the tire rotation rate, which is available over shorter cycles, compared to GPS signals. However, the distance factor is bound to change as the tires become worn, the tire air pressure changes or the tires are replaced, and furthermore, the sensor input can never be completely error-free. PTL1 discloses a method whereby the difference between a saved distance factor and a newly calculated distance factor is saved as error information if the difference is represented by a value equal to or greater than a specific value and upon detecting error information a predetermined number of times or more, a new distance factor is calculated based upon the error information.